For applications of silicon nanocrystals in memory device, it is necessary to understand the discharging behaviors of the nanocrystals as the data endurance in memory and decay constant are important parameters to be measured. In this work, we present a study on the charge trapping and charge decay in nc-Si embedded in SiO2 by electrostatic force microscopy (EFM). By using EFM technique, the amount of charge trapped in the nc-Si can be deduced based on the force arising from the Coulomb interactions between the charge of the sample and the charge on the tip. The charge amount and size of the charge cloud of the trapped charge are found to be seriously affected by nc-Si distribution in the SiO2 matrix.The influence of neighboring charges is studied with the creation of additional charge cloud close to the charge cloud under test. The size of charge cloud is obtained from pseudo-voigt fitting to the EFM resonance frequency.